1. Field of the Invention
The invention relates to a new manufacturing process for extruded bi-functional structural elements made of aluminium alloy, and structural elements produced by this process. The invention is particularly useful for the manufacture of weldable stringers for aeronautical construction.
2. State of the Art
Metal structural elements of an aircraft are currently assembled essentially by riveting. Welding is hardly used, because aluminium alloys with high mechanical properties such as Al—Cu—Mg type alloys in the 2xxx series and Al—Zn—Cu—Mg type alloys in the 7xxx series cannot be welded satisfactorily by fusion. On the other hand, most alloys in the 5xxx and 6xxx series can be welded by fusion. Among these alloys, the 6056 and 6156 alloys are used for aircraft structural elements, but more frequently they are assembled together by riveting. It is only recently that the fuselage skin made of a 6056 alloy has been welded onto extruded stringers made of a 6056 alloy, to assemble the fuselage of Airbus A318 and A380 aircraft. It is impossible at the moment to weld a fusion weldable alloy (such as 6056) to another alloy considered to be not weldable by fusion (such as 7349); the only way to make such an assembly in normal industrial practice is by riveting. According to the prior art, a welding assembly of two parts made from different alloys usually imposes restrictions on the choices of alloys. As far as the fastening of stringers and wink skin is concerned, this restriction concerns the alloy chosen for the stringers and for the skin. Taking into account these restrictions may necessitate optimizing a property balance if these alloys need to be assembled by welding, and preferably by fusion welding (such as MIG, TIG, laser welding) which is not possible for every alloy and notably not with all heat treatable alloys. As far as friction stir welding is concerned, it is possible to weld most aluminium alloys but with restrictions on the geometric configuration of the welded joint. The “property balance” as used in the art generally refers to a balance between the properties collected under the umbrella term as “static mechanical properties” (particularly the ultimate tensile strength UTS and the yield stress YS), on the one hand, and properties relating to “damage tolerance” (particularly toughness and resistance to fatigue crack propagation) on the other hand. However, even an optimized property balance generally provides structural elements which exhibit lower global performances. It would be advantageous in terms of cost and weight to be able to select an alloy with high mechanical properties for the stringers, an alloy with high damage tolerance for the wing skin and to weld them together. A solution for assembling such structural elements by welding can significantly simplify the assembly of an aircraft.